They’re still there, they were just hiding.
Your Soldiers will never be removed from your account unless you specifically contact us with that request.
What most likely happened is that your Soldier List was set to only display characters from one specific type.
There is a button that filters characters by army branch (Infantry, Tank Crewmen, Pilots, Recons, Paratroopers and Generals).
If you click on the small, diamond shaped icon under your character sidebar and choose to display all characters, you will see that every character is still there.